Smallest exoplanet mass measured

Appearing only as big as a golf ball seen from 10 million kilometres away, scientists have weighed the smallest planet yet discovered orbiting a distant star. The findings will help us to understand more about how our own Solar system was formed.

Writing in Nature this week, Pennsylvania State University astronomer Eric Ford and his colleagues have successfully established the size and mass of planet Kepler-138b. This tiny dot, which is 200 light years distant or about 10 million times farther away than our own Sun, orbits the red dwarf star Kepler-138.

It was discovered using data collected by NASA's Kepler space telescope, which spent 4 years gathering data from over 150,000 stars in one part of our Milky Way galaxy. Scientists are now combing through Kepler's data looking for telltale signs of planets orbiting the imaged stars.

What they are looking for are periodic dips in a star's luminoscity or brightness, which occurs when an orbiting planet passes in front of its host star. So far the technique has yielded over 4600 candidate so-called "exoplanets". The data from Kepler-138 showed that there were three planets in orbit, dubbed Kepler-138b, -c and -d, with the smallest, Kepler-138b taking just 10 days to complete a lap of the star and appearing, based on how long it blocked out the light from the host star, to be be roughly the same size as Mars.

The mass of large planets can be deduced by looking at how they warp the light coming from the star they are orbiting. As they circle, their gravity pulls the star towards the planet slightly, causing the issuing light to subtley change colour. Intuitively, a more massive planet exerts a more powerful effect on the star light than a smaller one.

But planets on the scale of Kepler-138b are too small to be assessed this way. Instead, the Penn. State team made painstaking measurements of how the three planets affect each other's orbits, enabling them to deduce that Kepler-138b has a mass only about 6.7% that of the Earth, or two-thirds the mass of Mars. This computes to a density of about 3 grams per cubic centimetre, which is consistent with a small, rocky world. And close in to the parent star that it is, the temperature is a sizzling 320 degrees Celsius. Its sister exoplanets, Kepler-138c and Kepler 138-d, weigh in at twice the size, and two-thirds the size of Earth, respectively.

Measuring the masses and densities of these distant worlds will help space scientists to refine the way that they model the formation of planetary systems, which often involve guesswork regarding how and where planets form, and how they jockey for orbital position, and how their ultimate location affects their composition, because planets close to their host star have a harder time hanging on to some of their lighter components, like hydrogen.

According to University of Calinfornia, Santa Cruz astronomer Simon Laughlin, who authored a commentary on the findings, also published in Nature this week, "It is imperative to improve our understanding of how our systemís architecture and evolution fit into the overall census -- the authorsí study is a step towards that goal."

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